1. Field of the Invention
This invention relates to methods and devices for measuring the dimensions and mechanical properties of pipes, tubes and the like. More particularly, it relates to an ultrasonic device which measures the wall thickness and the maximum and minimum diameter using an ultrasonic probe, instrumentation and a triangulated fixture. It includes subsequent calculations of the wall thickness eccentricity and the associated ovality [roundness] of the pipe and tube relative to position.
2. Description of the Related Art
A perfect circle is the most desirable cross section for tubulars. It provides the greatest strength—i.e., resistance to both internal and external pressure—of any known shape. Moreover, a perfectly [or near perfect] circular cross section facilitates the joining of tubulars to fittings, additional sections of tubular material and the like.
Wall thickness and wall integrity are also important parameters of tubular materials. In general, thicker walls and walls that are free from defects in the material forming the wall provide greater strength and hence greater safety. The combination of the wall thickness and associated ovality data along the length of the tubular is necessary calculations to determine collapse and burst pressures.
In the past, wall thickness and ovality have been measured separately, using two different measuring devices. Most commonly, wall thickness has been measured using an ultrasonic instrument with a transducer coupled to the tubular under test with a liquid [water] interface. The time for the ultrasonic waves to reflect from both the external and internal surfaces of the tubular is converted into distance (wall thickness) using the measured the time and the known velocity of the sound wave in the material.
Diameter is presently measured using a mechanical device such as a micrometer, an optical device such as a laser or camera or multiple ultrasound transducers or an array mounted to a known diameter fixture reference which surrounds the tubular.
An oval may be considered a flattened circle or an ellipse. An ellipse is a closed plane curve generated by a point moving in such a way that the sums of its distances from two fixed points (foci) is a constant. The length from one side of an ellipse to the other which passes through both foci may be considered its major diameter. A line perpendicular to the major diameter and passing through the ellipse at its widest part may be considered its minor diameter. Ovality may be defined as being the major diameter minus minor diameter. A perfect circle has an ovality of zero. Alternatively, pipe ovality may be expressed as a percentage:% Pipe Ovality=100×(Max. Diameter−Min. Diameter)/Mean Diameter
There are many useful applications for applying diameter, ovality, wall thickness and eccentricity determinations to pipe and tubes. Engineers use the combination of pipe wall thickness and ovality for their burst and collapse calculations The process of bending straight pipe to make it into a coil distorts its original, circular cross-section into an oval and leaves it with a permanent curvature. Codes prohibit installing pipe that is more than 5% oval. The diameter and ovality (shape) of pipe ends can greatly complicate aligning the ends for butt-welding the pipes together. Drill pipe is subjected to both outside and inside wear due to the environment Codes govern the dimensional properties of drill pipe.